A head-up display device using a projection technology to project display light from a display has been conventionally known. To protect the display from outside light such as sunlight, such a device may have a function to prohibit the outside light from entering (reaching) the display when intense outside light is detected during its use (for example, refer to a patent literature JP 2013-224068 A).
However, according to the study by the inventors of the present disclosure, the patent literature has no mention as to operation of the prevention mechanism when the head-up display device is stopped. Thus, when the head-up display is not operating, the display may expose to outside light such as sunlight for a long time, which may result in deteriorating the display.